Photodiodes are widely used for sensing light radiation. There are many applications in which the level of the light which is required to be sensed is very low, and therefore the sensitivity of said photodiodes is a critical requirement.
It is well known in the art that the signal-to-noise ratio which can be obtained from photodiodes (and from many other electronic components) is limited by the level of the “thermal noise”, which in turn is related to the temperature of the component. The term “dark current” is commonly used in the art to define the current flowing in a photodiode during a total dark condition. The signal-to-noise ratio in photodiodes is conventionally improved by cooling the component, in many cases down to very low temperatures close to 0° K. The means for cooling and maintaining such a low temperature in photodiodes, however, are cumbersome and expensive, and in any case can reduce the noise down to a limited value.
The dark current is generally composed of two main components. The first component, hereinafter referred to as “the diffusion dark current” is due to the thermal excitation of carriers across the complete energy bandgap of the photodiode material. As said, the level of this current can be reduced by means of cooling the component. The second component affecting the level of the dark current is known as the “Generation-Recombination” current (hereinafter “G-R dark current”). The level of the G-R dark current can also be reduced by cooling, but at a slower rate of reduction with temperature.
At low temperatures, where the level of the diffusion dark current is reduced sufficiently, the G-R dark current generally becomes the most dominant component of the dark current. There have been made many efforts in trying to reduce the level of the thermal noise. However, there are not known many of such efforts for reducing the G-R current.
FIG. 1 is a band diagram showing the principle of operation of a photodiode according to the prior art. In a semiconductor p-n junction 1-2, a depletion region 3 is formed around the metallurgical junction due to the transfer of electrons from donors in the n-side 2 of the depletion region to acceptors in the p-side 1. The conduction band (EC) and valence band (EV) are bent in the depletion region. This bending is associated with an electric field that drives electrons 7 towards the n-side and holes 8 towards the p-side of the junction. When a bias is applied to the junction, quasi Fermi levels can be defined in each of the two “flat-band” regions. The quasi Fermi level lies near the valence band on the p-side (EF(p)) and near the conduction band on the n-side (EF(n)). At zero bias, the energies of the two quasi Fermi levels are equal. The energy separation of the two quasi Fermi levels in electron-volts is equal to the applied bias in volts. If a reverse bias Vrev is applied to the diode, the following relationship holds:Vrev=EF(p)−EF(n).
The energy gap is given by EG=EC−EV. Although EC and EV change with position due to the band bending in the depletion region, their energy separation is constant everywhere for a “homo-junction” diode (“homo-junction” means that the same material is used on each side of the p-n junction).
Light 9 can be absorbed by promoting an electron 119 from the valence band to the conduction band. The missing electron in the valence band is called a hole, and is indicated by numeral 118. The longest wavelength for this process is called the cut off wavelength and is given by: λc=hc/EG, wherein h is Planck's constant and c is the velocity of light.
The “photo-created” hole 118 in process 9 exists in the n-type material 2 and so is a minority carrier. It can diffuse, as indicated by numeral 10 to the depletion region where it is accelerated 8 into the p-side 1 by the electric field in the depletion region 3. An analogous process (not shown explicitly) can occur in the p-type material 1 where a minority electron is created by the absorption of light. It can diffuse to the depletion region where it is accelerated 7 into the n-side 2 by the electric field in the depletion region 3.
Generation-Recombination (G-R) centers 4, also known as Shockley-Read traps or Shockley-Hall-Read traps, are energy levels that lie close to the middle of the band gap. They are related to imperfections or impurities inside the crystal. The probability of process 9 to occur due to heat (in the absence of an external photon flux) is essentially proportional to exp(−EG/kT) where k is Boltzman's constant and T is the absolute temperature. This process (and the equivalent process on the p-side) gives rise to the “dark current” in a perfect diode with no G-R centers. In this case the dark current is all due to diffusion dark current, and the device is said to be at “the diffusion limit”.
In an asymmetric p+-n homo-junction, where the p-doping is several orders of magnitude greater than the n-doping, it can easily be shown that, generally, in the diffusion limit, the higher of the two minority carrier concentrations, in said p+-n case the minority holes on the n-side, makes the dominant contribution to the dark current.
Since free electrons 7 and holes 8 are removed efficiently by the electric field in the depletion region 3, especially when a reverse bias is applied, an electron that undergoes excitation 5 from the valence band EV to the G-R center 4 cannot return to the valence band. It can only be further excited 6 to the conduction band. Processes 5, 6, 7, and 8 thus give rise to the G-R dark current.
The rate of electron generation by traps, in unit volume of the reverse biased depletion region 3 due to a process, 5, 6, 7, and 8, is approximately described by the formula
                    G        =                              n            i            2                                                              τ                                  n                  ⁢                                                                          ⁢                  0                                            ⁢                              p                ′                                      +                                          τ                                  p                  ⁢                                                                          ⁢                  0                                            ⁢                              n                ′                                                                        (        1        )            where ni is the so called intrinsic carrier concentration (the carrier concentration in the perfectly pure material) and τn0, τp0 are the electron and hole minority carrier lifetimes. This formula may be found, for example, as equation (8.9.2) in chapter 8 of the book by Shyh Wang, entitled “Fundamentals of Semiconductor Theory and Device Physics” (published by Prentice Hall, ISBN 0-13-344425-2). Here n′=n·e(Et−EF)/kT and p′=p·e(EF−Et)/kT where n, p, and EF are the electron concentration, the hole concentration and the Fermi level respectively in a given sample of the semiconductor material, Et is the energy of the trap, and T is the absolute temperature. It can be demonstrated that G in equation (1) is largest when the trap lies near the middle of the energy bandgap. In this case it is easy to show using the above formulae, that
                    G        ≈                              n            i                                (                                          τ                                  n                  ⁢                                                                          ⁢                  0                                            +                              τ                                  p                  ⁢                                                                          ⁢                  0                                                      )                                              (        2        )            
Under these conditions the G-R contribution to the dark current can become significant. From equation (2) it follows that G is then proportional to the intrinsic carrier concentration, the formula for which contains an exponential factor: exp(−EG/2kT). The dark current due to generation-recombination centers which is itself proportional to this value of G will thus also vary essentially as: exp(−EG/2kT). It is the weaker temperature dependence of the G-R contribution to the dark current (exp(−EG/2kT)) compared with the diffusion contribution (exp(−EG/kT)) that causes the G-R contribution to dominate at low temperatures. The ratio of the G-R dark current to the diffusion dark current in a p+-n diode is often approximately given by equation (8.9.6) in chapter 8 of the earlier mentioned book by Shyh Wang, as:
                                          J                          G              -              R                                            J            diff                          =                                            L              dep                                      L              p                                ×                                    N              D                                      n              ′                                                          (        3        )            where Ldep is the thickness of the depletion region, and ND and Lp are the doping and minority carrier diffusion length on the n-side of the junction. Typical values of Ldep and Lp are ˜0.5 μm and 20 μm respectively.
Typical narrow gap homo-junction photo-diodes based on e.g. InSb, InAsSb, HgCdTe, etc., are in many cases operated at reduced temperatures, in order to limit the dark current. For such devices operated at 77K, G-R centers typically increase the dark current above the diffusion limit by at least 3-4 orders of magnitude in the MWIR (3-5 μm) and 1-2 orders of magnitude in the LWIR (8-12 μm) cut-off wavelength regions, behaviour that in each case is consistent with equation (3). This effect may easily be seen in J Bajaj, SPIE proceedings no. 3948 page 45 (FIG. 3 of this article), San Jose, January 2000, or in P C Klipstein et al., SPIE proceedings number 4820, page 653 (FIG. 2 of this article), Seattle, July 2002.
Until recently, the prior art has failed to specifically address the issue of suppressing the G-R contribution to the current by a suitable hetero-junction design. Previous attempts at suppressing the dark current have focused either on reducing the diffusion current from flat band regions close to the depletion region, or by suppressing the Auger contribution in devices operating close to room temperature. These works are summarized in the “Background of the Invention” section of WO 2005/004243 by the current applicant.
To our knowledge the only two works that do refer to the suppression of the G-R contribution to the dark current at low temperatures (where it is usually dominant) are the said WO 2005/004243 and an abstract entitled “InAsSb/GaAlSb/InAsSb nBn detector for the 3-5 μm” by S Maimon and G W Wicks that appeared in the book of abstracts of the 11th international conference on Narrow Gap Semiconductors (NGS-11) which took place in Buffalo, Jun. 16-20, 2003.
In the abstract of Maimon and Wicks a unipolar device is proposed exhibiting a suppressed G-R current, comprising two narrow bandgap n-type layers surrounding a wide bandgap barrier layer, in which there is negligible offset between the valence bands of the wide and narrow bandgap materials. It is claimed there that the device has no depletion region, but no information is disclosed on how this can be achieved when a bias is applied.
FIG. 2a illustrates a bipolar device according to the said WO 2005/004243 in which a “depletion-less” 29 photon absorbing region 13 is created within a p-n diode by inserting a suitable large bandgap barrier layer 14 between the n-type photon absorbing layer 13 and the p-type contact layer 15. The doping of this barrier layer 14 is critical in order to achieve a “depletion-less” photon absorbing region 13, and is most conveniently doped n-type, as in the example shown in FIG. 2a. In FIG. 2a the donor concentration in the barrier layer 14 has been tailored so that the p-type contact layer 15 may be biased to raise its valence band above that in the photon absorbing layer while at the same time maintaining entirely flat bands in the photon absorbing layer 13. In FIG. 2b, the band diagram is shown at a slightly lower bias, where the bands of the photon absorbing layer 13 next to the barrier layer are accumulated 21. Even in this case there is no depletion in the photon absorbing layer 13 and provided the lowest part of the valence band in the barrier layer 14 is not more that ˜10kTop below the flat part of the photon absorbing layer 13, minority carriers created by photons absorbed in the photon absorbing layer 13 can be thermally excited across the barrier layer 14 and arrive in the contact layer 15 where they constitute a photocurrent which then passes through an external monitoring circuit. In both FIGS. 2a and 2b the only depleted regions 28 are those with bandgaps at least twice that of the photon absorbing layer 13. Hence, as described above, any G-R currents that they produce have a higher activation energy than that for the diffusion current produced in the photon absorbing layer 13 and so may be regarded as negligible. The device thus exhibits a dark current characteristic of a diffusion limited homojunction diode whose bandgap is similar to that of the photon absorbing layer 13. The examples in FIGS. 2a and 2b are based on a InAsSb photon absorbing layer 13, a GaAlAsSb barrier layer 14 and a GaSb contact layer 15. Other examples from WO 2005/004243 based on a GaInSb/InAs superlattice photon absorbing layer 33 are shown in FIGS. 2c and 2d. In FIG. 2d note that the bandgap of the superlattice contact layer 45 is similar to that in the photon absorbing layer 33. Any electrons generated by G-R centres in the contact layer 45 are blocked from transfer to the photon absorbing layer 33 by the tall barrier 34. They will eventually recombine with holes in the contact layer 45 and do not contribute to the current.
All of the previous devices discussed above, apart from that in Wicks and Maimon abstract, are BIPOLAR photodiodes with contacts to both n-type and p-type regions. All including that in Wicks and Maimon abstract are sensitive to a single wavelength band. It is an object of the present invention to provide a UNIPOLAR photosensitive device, sensitive to up to TWO wavelength bands, in which the dark current is significantly reduced, particularly at low temperatures, generally in the range of about 77 to 200° K., depending on the material and wavelength of operation. Advantages of the unipolar device over the bipolar one can include greater simplicity of manufacture of both single color and two color devices.
It is a particular object of the present invention to provide a photosensitive device in which the level of the G-R current is significantly suppressed at a given temperature.
It is still an object of the present invention to reduce the need for cooling, by providing a photosensitive structure having a level of dark current that would alternatively exist in a standard bipolar photodiode at much lower temperature.
It is still a further object of the invention to provide a method and process for manufacturing the photosensitive structure of the present invention.
Other objects and advantages of the present invention will become apparent as the description proceeds.